1. Field of the Invention
The present invention relates to a quartz crystal oscillator in which at least a quartz crystal unit and an oscillation circuit using the quartz crystal unit are accommodated in a surface mount type container, and more particularly, to a crystal oscillator that uses a container body with a recess on each of its opposite principal surfaces such that the container has H-shaped cross section and that has a terminal used for test and/or adjustment on the outer side surface of the container body.
2. Description of the Related Art
A surface mount type crystal oscillator is characteristically small and lightweight, and a surface mount type temperature compensated crystal oscillator, among others, that incorporates a temperature compensation mechanism for compensating the frequency-temperature characteristics of the crystal unit has characteristically a good frequency stability against temperature changes. The surface mount type crystal oscillator, therefore, is widely used as a reference source of frequency and time, particularly in portable electronics devices, including a mobile telephone. In recent years, as the portable electronics devices are reduced in their size, there has been a need for a smaller surface mount type crystal oscillator.
FIG. 1A is a sectional view showing an exemplary configuration of a conventional surface mount type temperature compensated crystal oscillator. The crystal oscillator is shown composed of container body 1, quartz crystal blank 2, and IC (integrated circuit) chip 3. When container body 1 is mounted on a wiring board, it appears as oblong box with long sides and short sides when viewed from above, assuming an appearance of flat and a generally rectangular shape. Each of the top and lower surfaces is provided with a recess, resulting in H-shaped cross section. Such container body 1 consists of a laminated ceramic including generally rectangular and planar center layer 1a, and upper and lower frame layers 1b, 1c each having an opening. Each of frame layers 1b, 1c is formed in a frame shape which has wall portions corresponding to the respective sides of center layer 1a and the opening surrounded by the wall portions. Center layer 1a and frame layer 1b form first recess 20a shown in the upper part of the view, and as described below, crystal blank 2 that functions as a crystal unit is accommodated in first recess 20a. Similarly, center layer 1a and frame layer 1c form second recess 20b shown in the lower part of the view, and IC chip 3 incorporating an oscillation circuit and a temperature compensation mechanism is accommodated in second recess 20b. In the figure, although any of center layer 1a and frame layers 1b, 1c is shown composed of a single layer for convenience, any of center layer 1a, and frame layers 1b, 1c is actually composed of a plurality of ceramic layers. In particular, center layer 1a is composed of first layer A as shown in the upper part and second layer B as shown in the lower part, and shield electrode layer 4 made of a metal film is provided on an interface between layers A, B.
Although the crystal oscillator described herein is a so called two-chamber type, which contains the crystal blank and the IC chip in respective recesses or chambers which are separately provided, there is alternatively a one-chamber type crystal oscillator in which both the crystal blank and the IC chip are accommodated within the same space of the container body. In general, an assembly in which a crystal blank is contained in a container referred to as a crystal unit.
Crystal blank 2 is, for example, composed of an AT-cut quartz crystal blank having a generally rectangular shape, and has an excitation electrode on each of its opposite principal surfaces. An extension electrode is extended from each of the pair of excitation electrodes toward opposite sides of an end of crystal blank 2. A pair of crystal retaining terminals 6 is provided on the bottom surface of first recess 20a, or the top surface of center layer 1a. Crystal blank 2 is fixed and retained in first recess 20a at a location where the pair of extension electrodes is drawn, by adhering each of these extension electrodes to crystal retaining terminal 6 with, for example, conductive adhesive 7.
After crystal blank 2 is fixed, metal cover 11 is joined, at the opening face of first recess 20a of container body 1, onto a metal ring (not shown) provided on the top surface of frame layer 1b by, for example, seam welding, so that crystal blank 2 is hermetically sealed within first recess 20a. 
FIG. 1B is a bottom view of the crystal oscillator shown in FIG. 1A, which is not mounted with IC chip 3, and FIG. 1C is a partial sectional front view of the crystal oscillator shown in FIG. 1A, partially showing the outer side surface of container body 1.
The side surface of container body 1 is provided with four write terminals 10 for writing temperature compensation data to the temperature compensation mechanism. Write terminals 10 are arranged such that two terminals are provided on each long side of generally rectangular center layer 1a. 
In container body 1, mounting terminal 5, which is used to surface-mount the crystal oscillator on a wiring board, is formed at each of four corners of frame layers 1c that form second recess 20b. There are provided four mounting terminals 5; a power terminal, a ground terminal, an output terminal on which oscillation output appears, and an AFC (automatic frequency control) terminal to which an AFC signal is supplied. Each mounting terminal 5 is composed of bottom electrode 5a formed on the top surface of frame layer 1c, or a face that would contact with a wiring board, and side surface electrode 5b provided by a portion of bottom electrode 5a extending over the outer side surface of container body 1. At this time, each of side surface electrode 5b is formed on opposite ends of each side surface including each of the long sides of container body 1, among four side surfaces of container body 1.
A method of forming such side surface electrode 5b and write terminal 10 will now be described. To form a surface mount type container body made of a laminated ceramic, ceramic sheets each having a size corresponding to multiple container bodies are typically used, stacked, baked, and then divided into an individual container body. Each ceramic sheet corresponds to a ceramic layer described above. In this case, ceramic sheets for the layers having a size corresponding to multiple container bodies 1 are also used. Side surface electrode 5b and write terminal 10 are formed by so called through-hole processing when container body 1 is formed with the laminated ceramic, after electrode patterns made of tungsten (W) or the like are printed on ceramic sheets for the layers and the ceramic sheets for the layers are integrated. At this time, side surface electrode 5b is formed to be located on an end surface of each of ceramic layers except the outermost ceramic layer on a first recess 20a side, that is, the topmost ceramic layer, in order to prevent side surface electrode 5b from electrically short circuiting with metal cover 11. Write terminal 10 is formed to be located on an end surface of each ceramic layer except respective outermost ceramic layers on the top and bottom sides of the laminate, thereby preventing write terminal 10 from electrically short circuiting with metal cover 11 or a wiring board. When through-hole processing is provided, even a ceramic layer to be void of any side surface, electrode 5b or write terminal 10 is also provided with a hole penetrating the layer to form a through-hole surface. The ceramic sheets are stacked on top of another while 5 providing such through-hole processing as described above. Subsequently, the laminate is baked, subjected to, for example, gold plating on electrode patterns, and then divided into an individual container body 1. In this way, container body 1 with side surface electrode 5b and write terminal 10 is formed.
As shown in FIG. 1B, a plurality of circuit terminals 8 are arranged on the bottom surface of second recess 20b, or the back surface of center layer 1a, of container body 1 along both long sides of center layer 1a. Circuit terminals 8 are provided corresponding to IC terminals provided on IC chip 3 as described below. In this figure, respective five circuit terminals 8 for each long side are arranged in line. Four of these circuit terminals 8 are power, ground, output and AFC terminals corresponding to mounting terminals 5 described above, respectively, and electrically connected to corresponding mounting terminals 5 through conductive paths formed in center layer 1a. The remaining two of circuit terminals 8 are crystal circuit terminals 8a electrically connecting with crystal blank 2. Crystal circuit terminal 8a is connected to crystal retaining terminal 6 provided on the bottom surface of first recess 20a through crank-shaped via-holes 9a, 9b or the like provided in center layer 1a of container body 1. The remaining four circuit terminals except circuit terminals 8 corresponding to mounting terminals 5 and crystal circuit terminals 8a are write circuit terminals, and each of the write circuit terminals is electrically connected to write terminal 10. In addition, a pair of crystal test terminals X1, X2 is provided on the back surface of center layer 1a. Crystal test terminals X1, X2 are electrically connected to crystal circuit terminals 8a through linear conductive paths provided on the back surface of center layer 1a. Accordingly, crystal test terminals X1, X2 are electrically connected to crystal blank 2.
IC chip 3 is generally rectangular. In IC chip 3, an oscillation circuit using crystal blank 2, and a temperature compensation mechanism for compensating the frequency-temperature characteristics of crystal blank 2 are integrated on a semiconductor substrate. The oscillation circuit and temperature compensation mechanism are formed on one principal surface of the semiconductor substrate by typical semiconductor device fabrication processes. Of opposite principal surfaces of IC chip 3, then, the one on which the oscillation circuit and temperature compensation mechanism are formed on the semiconductor substrate will be referred to as a circuit-forming surface. A plurality of IC terminals 7 are arranged on the circuit-forming surface along both long sides thereof. The IC terminals corresponds to circuit terminals 8, 8a on center layer 1a of container body 1, respectively. The IC terminals are used for electrical connection of a circuit in IC chip 3. These IC terminals include terminals connected to crystal circuit terminals 8a for electrically connecting crystal blank 2 to the oscillation circuit, a power terminal, an output terminal, a ground terminal, an AFC terminal, and terminals for writing temperature compensation data. As such an IC chip, for example, type AN28518 available from Matsushita Electric Industrial Co., Ltd. may be used. IC chip 3 is secured to the bottom surface of second recess 20b by using so called flip-chip bonding technique to bond the IC terminals to circuit terminals 8, 8a provided on the bottom surface of second recess 20b by ultrasonic welding using bumps 18. Incidentally, shield electrode layer 4 embedded in center layer 1a is for electrically shielding the excitation electrodes of crystal blank 2 from IC chip 3.
To fabricate such a surface mount type temperature compensated crystal oscillator, crystal blank 2 is first accommodated and hermetically sealed in first recess 20a to constitute a crystal unit, and thereafter, vibration characteristics, such as crystal impedance (CI), or temperature characteristics of crystal blank 2 5 as a crystal unit is measured using a pair of crystal test terminals X1, X2 provided on the bottom surface of second recess 20b, or the back side of center layer 1a. Specifically, a probe is brought into contact with crystal test terminals X1, X2 to measure the vibration characteristics or temperature characteristics. If any vibration characteristics or temperature characteristics is found to be abnormal, the unit will be discarded as a defective. With an accepted unit, IC chip 3 is mounted to the bottom surface of second recess 20b and temperature compensation data is written into IC chip 3 from write terminals 10 provided on the outer side surface of container body 1. A probe is brought into contact with side surface electrodes 5b in mounting terminals 5 to check the oscillation characteristics of the crystal oscillator. Finally, although not shown herein, resin is injected in second recess 20b as an “underfill” to protect the circuit-forming surface of IC chip 3, and the temperature compensated crystal oscillator is thus completed. Incidentally, because each of side surface electrodes 5b and write terminals 10 is formed on a through-hole surface running through in the vertical direction of container body 1, there is no protrusion that causes an obstruction when a probe is brought into contact: this facilitates the contact of the probe.
In a surface mount device (SMD), because the presence of a solder fillet can be referenced to accurately check whether or not soldering is acceptable, a side surface electrode is typically provided in addition to a bottom electrode in a mounting terminal, and the crystal oscillator described above uses a side surface electrode as a measurement terminal for the oscillation characteristics. This is because a mounting terminal thus formed allows a probe to contact with both the bottom and side surfaces in consideration of a jig and the like, facilitating measurements. In addition, side surface electrode 5b can be used to check the oscillation characteristics of the crystal oscillator even after mounted on a wiring board.
Although the temperature compensated crystal oscillator described above includes four write terminals 10 for writing temperature compensation data, the number of write terminals is not limited to this, and only two write terminals may be provided on an outer side surface of container body 1, depending on a circuit design of the temperature compensation mechanism.
However, in the temperature compensated crystal oscillator described above, crystal test terminals X1, X2 provided on the bottom surface of second recess 20b of container body 1 will be covered by IC chip 3 after IC chip 3 is fixed to second recess 20b, as well as covered by resin for an underfill. As a result, after completed as a product, the crystal unit could not solely be measured for the vibration characteristics. In this case, even if there is any failure such as an oscillation failure in the temperature compensated crystal oscillator after shipment, it would be difficult to ascertain the cause of the oscillation failure due to the inability of the crystal unit to be solely checked to analyze the vibration characteristics.
In addition, when the size of a temperature compensated crystal oscillator is further reduced, for example, to or below a geometry of 3.2×2.5 mm, the bottom dimensions of the recess of container body 1 is reduced accordingly. This will make it difficult to form crystal test terminals X1, X2 having a sufficient size on the bottom surface of second recess 20b. For example, in consideration of a probe on an instrument for measuring the oscillation characteristics, each of crystal test terminals X1, X2 require to be 0.6×0.6 mm or larger. However, further reduction in size of the temperature compensated crystal oscillator will reduce the size of the crystal test terminals to or below this size, making it difficult to reliably measure the vibration characteristics. In addition, because crystal test terminals X1, X2 are disposed between rows of circuit terminals 8, there is a problem that the miniaturization of IC chip 3 itself could not be addressed.
Accordingly; it is conceivable that in a two-chamber type temperature compensated crystal oscillator as described above, crystal test terminals are arranged on the outer side surface of the container body, similar to a one-chamber type temperature compensated crystal oscillator (for example, see US 2006/0055478 A1). FIG. 2 shows an exemplary configuration of a one-chamber type temperature compensated crystal oscillator. In this crystal oscillator, a single recess is formed in container body 1 and a step is formed in the recess. IC chip 3 is fixed to the bottom surface of the recess, and crystal blank 2 is fix to a pair of crystal retaining terminals 6 provided on the top surface of the step with conductive adhesive 7. Crystal test terminals X1, X2 are provided on the outer side surface of container body 1, and each of crystal test terminals X1, X2 is electrically connected to a pair of crystal retaining terminals 6 through a conductive path 12 formed an interface between ceramic layers in container body 1. In this case, crystal retaining terminal 6 is electrically connected to crystal circuit terminal 8a formed on the bottom surface of the recess through via-hole 9a formed in the step on the inner wall. The thickness of conductive path 12 is smaller than that of crystal retaining terminal 6.
If the configuration as shown in FIG. 2 that has crystal test terminals arranged on the outer side surface of the container body was simply applied to a two-chamber type crystal oscillator as shown in FIGS. 1A to 1C, crystal retaining terminals 6 would be electrically connected to crystal test terminals X1, X2 on the outer side surface of container body 1 through a conductive path 12 formed on an interface between center layer 1a and frame layer 1b located above, as shown in FIG. 3. However, as the size of a crystal oscillator is reduced, the width of the frame potion of frame layer 1b of container body 1 needs to be reduced accordingly, thereby resulting in a reduced through-path of conductive path 12 on the interface. Consequently, when an impact is applied, cracks may occur on container body 1 near conductive path 12 and cause failure of air tightness that would be a crucial defect for a crystal unit. This is due to the fact that, in a laminated ceramic, the interface shows a higher bonding strength without a conductive path, or an electrode film. The width of a frame portion refers to a distance between the surface toward the recess side and the outside surface of the frame layer, in the direction from the recess of the container body to the outer side surface.
In addition, in the configuration shown in FIG. 2, it must be necessary to consider how crystal test terminals are arranged when they are provided on the outer side surface of container body 1. If there are two write terminals 10 that have been provided on the outer side surface of container body 1, constraints would be less on the arrangement of the crystal test terminals. However, if there are four write terminals 10, that is, if two write terminals 10 are provided on each side surface including the long side of container body 1, any crystal test terminal can no longer be provided on the side surface. In this case, two side surfaces including the short sides of the container body among the side surfaces of container body 1 are taken to provide one crystal test terminal on each side surface. FIGS. 4A and 4B show a temperature compensated crystal oscillator having thus arranged crystal test terminals X1, X2. However, again in this case, it is necessary to increase the width of the outer wall portion of the container body to ensure the strength of the container body, and because a step must be provided in the recess, it is difficult to reduce the outside dimensions of the container body. In the configuration shown in FIGS. 4A and 4B, even when crystal test terminals X1, X2 are provided on the short side of the container body, two crystal test terminals X1, X2, four write terminals 10, and four end surface electrodes 9b, or a total of 10 electrodes or terminals, need to be provided on the outer side surface of container body 1: this may prevent a crystal oscillator from being further reduced in size to, for example, an outer geometry of 2.0×1.6 mm.